DC machines, including DC generators and DC motors, are known. One type of DC machine is a brushless DC motor. Brushless DC motors are sometimes referred to as brushless permanent magnet motors, synchronous permanent magnet motors, or electronically commutated DC motors. Brushless DC motors are used for many applications, including applications in the automotive field.
A brushless DC motor has a rotor with permanent magnets and a stator with windings. In brushless DC motors the commutation of the stator windings is performed electronically based on the rotor position. Thus, there are no brushes or mechanical commutator, and instead control electronics are used to energize the stator windings synchronously. The stator windings are switched on and off in sequence to create a rotating magnetic field around the stator, which creates torque to pull and rotate the rotor.
There are several advantages in brushless machines over brushed machines—there are no sparks from a brush, and brush-life, brush residue and noise issues are either nonexistent or mitigated. Brushless DC machines can be faster, more efficient, reliable and quiet than DC machines with brushes. However, brushless DC machines require electronic commutation control to energize each stator winding at the right time.
To properly control the energization of the stator windings, sensors associated with the rotor are used to signal electronics to control switching elements. The sensors sense the rotor position relative to each stator winding. The current in the stator windings is controlled in frequency and phase angle to maintain a constant angular displacement between the poles of the rotating stator field and the rotor field poles. Such a constant angular displacement also exists in many other DC machines, such as DC generators.
The stator current may be either bipolar or unipolar. Bipolar may be reversible rectangular waves or sinusoidal; unipolar is usually a rectangular wave, with no current reversal.
The control electronics that energizes the stator windings must know the instantaneous rotor position relative to each stator winding. As stated above, this is done with sensors. However, determining the relative position between a rotor pole and a stator pole cannot be done unless the initial mounted position of the rotor sensors relative to each stator winding is known. Thus, the accuracy of the positioning of the sensors is a critical aspect of a brushless DC motor. This positioning affects the current flowing through the stator windings, the torque produced in the motor, the losses, the efficiency, and the stress on power electronic components.
Several types of sensors have been used to determine rotor position, such as absolute position sensors, single-bit position sensors, or sensors that measure the stator winding's back EMF (electromotive force) zero crossing points, or a combination thereof. To sense a stator winding's back EMF, the winding that is being measured cannot be energized. This means that, in a three-phase motor that uses back EMF for this purpose, only two phases will have current flowing through them at any given time.
Absolute position sensors are more expensive than, and relatively undesirable compared to, single-bit position sensors. Single-bit position sensors can be magnetic (such as Hall sensors) or optical (such as an optical shaft encoder), are often less expensive than absolute position sensors, and are more reliable than back EMF sensors since back EMF sensors do not work at speeds close to zero.
However, a single-bit position sensor must be mounted in precise alignment with the stator windings. Accurate alignment of a single-bit position sensor requires specialized production equipment and labor, which increase production costs. The alignment can be affected by factors such as mechanical wear of production equipment, deviation of settings, etc.
To improve the drawbacks of a single-bit position sensor, a novel process is presented, which can automatically correct for mounting position error of single-bit position sensor.
Accordingly, there is a need to provide a method and system for correcting rotor sensor mounting error in brushless DC machines.